1. Field of the Invention
The present invention relates to an apparatus for positioning a working table on which a work piece is mounted for machining, assemblage, testing and the like in, especially, machine tools, assembly plants, test equipment, semiconductor fabricating apparatus and the like.
2. Description of the Prior Art
An apparatus for positioning a working table on which a work piece is mounted is well known as means that is primary and general applicable for promoting the recently remarkable development in mechatronics technology. Apparatus of this kind has been conventionally used in a working table in a machine tool to vertically move to a predetermined position a work piece that is mounted on the working table to be machined.
In recent years, the use of such apparatus extends into many technical fields, for instance, transfer apparatus, industrial robots, measuring instruments, semiconductor manufacturing apparatus having assembling units, testing instruments and exposure units, and the like. It is further well known that such apparatus may be mounted on a X-Y positioning table to be thereby used as a three-dimensional X-Y-Z positioning table. Shown in Japanese Utility Model Application Laid-Open No. (Hei) 6-53042 is a positioning table that may be moved vertically in virtue of an inclined plane. The table unit comprises a base, a first table having an inclined plane on its top face and being movable in the horizontal direction, a first driving mechanism being of a threaded shaft which is provided on the base to travel the first table, a second table being movable along the inclined plane, and a second driving mechanism being of a threaded shaft which is provided on the first table to travel the second table.
In the positioning table unit cited in the above, the threaded shafts are each provided for each of the tables to bear the loads so that two sets of driving mechanisms are required whereby the table unit adversely becomes too big in height. The prior positioning table has drawbacks in which it is in particular difficult to secure the required rigidity and in which the tables are unstable in positioning and operation. Accordingly, the positioning table unit is unsuited for the working table in the machine tool, in which the precision positioning is inevitable.
Japanese Patent Application Laid-Open No. (Hei) 4-2406 discloses a positioning table unit having a lifting table secured to the top of a threaded ball screw shaft which is arranged in vertical direction. The threaded ball screw shaft has therein a center hole in which a spline shaft fixed to a stationary bed is fitted in such a manner that it is movable in axial direction but restrained in rotational direction. The thread ball screw shaft is screwed on a nut that is directly connected with a rotating member of a driving source provided on the stationary bed so that the lifting table may be moved vertically.
Proposed in the above Laid-Open No. (Hei) 4-2406 are alternative embodiments of a bar system and a pantograph system in which the horizontally operated variable may be converted into the vertical displacement.
The lifting table disclosed in the above Laid-Open No. (Hei) 4-2406 has the threaded ball screw shaft arranged in vertical direction, whereby the table unit adversely becomes too big in height. The big load is substantially born by rolling elements engaged with the treaded ball screw shaft so that the threaded ball screw shaft should be made large in diameter, resulting in the large scale of the positioning table unit. Thus, the prior art is also unsuited for the working table which is to be moved vertically under the situation where the work piece is supported thereon.